Hi Jklen,
As far as I know there are only
minimum age requirements for touring the glacier which I believe is eight years old.
As far as Ushauia and Iguazu hotels go, you could check Tripadvisor. There are 44 revivews for
hotels in Ushuaia and 20 reviews for
hotels in Puerto Iguazu . The good thing about Tripadvisor reviews is that loads of people submit reviews so there's a good breadth of opinion.
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I am also wondering if it's easy to book tours on the fly instead of prearranged.
It is definitely the busier season in those parts. I'm pretty sure you shouldn't have a problem booking a tour on the fly, but you may not get exactly what you want, just be aware of that. If you're really looking forward to a certain tour in particular, you may want to look into booking ahead of time....